Refractory metal silicide technology has been recognized as one of the keys to realizing good device performance in integrated circuits as device dimensions scale down. Titanium disilicide (TiSi.sub.2) has become recognized as one of the most attractive metal silicides, because of its low resistivity, stability, and capability for self-aligned formation.
One of the major advantages of titanium disilicide technology is the availability of a self-aligned VLSI process. That is, by depositing a layer of titanium metal overall and then heating in a nitrogen atmosphere, all exposed areas of silicon (whether monocrystalline or polycrystalline) will react to form titanium disilicide. A composition dominated by titanium nitride will be formed where the titanium metal was not in contact with silicon but was instead in contact with silicon dioxide. This is tremendously useful, since, silicide may be formed on the surface of exposed source/drain regions (or other exposed substrate surface regions), on the surface of the polysilicon gate level, and nowhere else. This means that the source/drain diffusions can be made shallower while still preserving an acceptably low sheet resistance. Also the sheet resistance of an interconnection to a polysilicon gate can be lowered. The use of nitrogen atmosphere in this process is critical, since otherwise silicon would out-diffuse through the growing silicide layer and permit lateral growth. The titanium disilicide might thereby bridge gaps of approximately 0.5 .mu.m, e.g., between the gate and source/drain of a VLSI device.
The second advantage of the titanium disilicide technology is the resulting titanium nitride (TiN) layer. This layer may be advantageously used as an interconnect layer between devices and additionally as a diffusion barrier for the underlying silicon or doped silicon region during subsequent processing steps.
Present refractory metal silicide processes, however, are being pressed by new circuit goals. In particular, 1.0 .mu.m and sub-micron circuits imply shallow diode junctions. An overlaying silicide layer must also be shallow to be compatible with a particular diode junction. Otherwise, a silicide layer might spike through the shallow diode junction, shorting the interconnect of which it is a part to the semiconductor substrate below.
Also, thick silicide layers place undesirable mechanical stresses on a junction. These stresses reduce the reliability of the circuit. Current silicide methods first deposit a titanium layer on a substrate. The layer is typically 1000 Angstroms thick. Nitrogen at a temperature of 500.degree. to 700.degree. C. and at atmospheric pressure is then heated with the titanium layer. These processes typically produce 1,500 Angstroms of titanium disilicide and 400 Angstroms of titanium nitride for each 1,000 Angstroms of titanium deposited.
New 1.0 .mu.m and sub-micron processes require the titanium disilicide layer to be only 500 Angstroms thick. This requirement implies that the deposited titanium layer be one-third the thickness deposited by older processes, or approximately 333 Angstroms thick initially. Unfortunately, refractory metals such as titanium are not easily deposited to such small thicknesses.
Alternatively, titanium may be reacted with nitrogen at a pressure less than one atmosphere. This process produces the desired higher ratio of titanium disilicide to titanium nitride. Unfortunately, this process produces almost no titanium nitride which is also desired in applications calling for a local interconnect.
Therefore, a need has arisen for a refractory metal-silicide process which is able to produce a thin silicide layer compatible with sub-micron processes yet which also produces a nitride layer for use as an interconnect.